trace.c: ensure NULL is not passed to printf

GNU printf, and many others, will print the string "(null)" if a NULL
pointer is passed as the argument to a "%s" format specifier.  Some
implementations (like on Solaris) do not detect a NULL pointer and will
produce a segfault in this case.

So, fix this by ensuring that pointer variables do not contain the value
NULL.  Assign the string "(null)" to the variables are NULL.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace.c

@@ -154,6 +154,7 @@ static const char *quote_crnl(const char *path)
 /* FIXME: move prefix to startup_info struct and get rid of this arg */
 void trace_repo_setup(const char *prefix)
 {
+	const char *git_work_tree;
 	char cwd[PATH_MAX];
 	char *trace = getenv("GIT_TRACE");
 
@@ -164,8 +165,14 @@ void trace_repo_setup(const char *prefix)
 	if (!getcwd(cwd, PATH_MAX))
 		die("Unable to get current working directory");
 
+	if (!(git_work_tree = get_git_work_tree()))
+		git_work_tree = "(null)";
+
+	if (!prefix)
+		prefix = "(null)";
+
 	trace_printf("setup: git_dir: %s\n", quote_crnl(get_git_dir()));
-	trace_printf("setup: worktree: %s\n", quote_crnl(get_git_work_tree()));
+	trace_printf("setup: worktree: %s\n", quote_crnl(git_work_tree));
 	trace_printf("setup: cwd: %s\n", quote_crnl(cwd));
 	trace_printf("setup: prefix: %s\n", quote_crnl(prefix));
 }